Hidden forums hasn’t an admin page and you must edit the php file. Locate your “hidden-forums.php” file. On top you will see a couple of line that you can change together with a brief documentation regarding the changes you can make. Remember that with this plugin you can hide forums but not individual posts.

I think that Hidden Forums can be modified in order to hide individual posts. It’s just a matter of changing the hidden_forums_filter filter function but it seems that the author doesn’t update her plugin no more.

The tricky thing with Hidden Forums is the lack of a frontend. The plugin doesn’t have a frontend at all… there are only manual settings which let you set individual forums as hidden or not. The frontend is handled through bbPress’ built-in “move topic” pulldown…

That said, someone might be able to hack Hidden Forums to add an extra frontend toggle that lets admins hide individual topics!